The field of the invention is closed loop control systems, and particularly, control systems in which the reference input is a sinusoidal signal having a wide frequency range and in which a regulator is employed to produce a control signal by summing this reference input with a feedback signal.
There are numerous applications for closed loop control systems in which a regulator receives a reference input signal and produces a control signal for a plant. For example, in a variable speed motor drive a sinusoidal current command signal is input to a regulator and summed with a motor stator current feedback signal to produce an error, or control signal, which operates an inverter to produce the commanded motor current. The control signal produced by such a regulator typically is a proportional plus integral signal (i.e. "PI") which includes a first component that is proportional to the difference between the sinusoidal reference input and the feedback signal and a second component that is proportional to the integral of that difference.
While proportional plus integral regulators work quite satisfactorily over a narrow frequency range, they are inaccurate when operated at the upper end of a wide frequency range of reference input signals. This inaccuracy is manifested as a control signal which does not have the proper amplitude and phase at higher frequencies with the result that the controlled plant does not operate as commanded by the reference input.